1. Field of the Invention
The present invention in one aspect relates to an apparatus for supplying powder together with a gas and, more particularly, to an apparatus which can supply a trace amount of powder at a constant rate. Still more particularly, the present invention is concerned with a powder supplying apparatus capable of supplying, with time variation of the Supply rate greatly suppressed, a trace amount of a fine powder to, for example, an apparatus which is used for spraying powder particles.
The present invention in its another aspect relates to a powder spraying apparatus which is used for, example, in spraying powder particles of an extremely small particle size ranging from several .mu.m to 10 .mu.m into a space between a pair of transparent substrates which form a liquid crystal panel so that a uniform gap is formed between these substrates by the powder particles which serve as spacers.
2. Description of the Related Art
There is an increasing demand for an apparatus which can continuously or consistently supplying, at a constant rate, inorganic or organic fine powder particles such as of a metal, ceramics or a plastic, as well as for an apparatus which can uniformly spray such particles. Such demand exists in various industrial or technical processes such as a plasma spraying process, processes for producing a liquid crystal device, powder compressing process, sand blasting process or powder coating process.
A description will now be given of a process which is employed in production of a liquid crystal and which is a typical example of the processes which require the constant supply and uniform spraying of fine powder particles. A liquid crystal display panel of a liquid crystal display device has a pair of transparent substrates. A liquid crystal is charged so as to fill the gap between these substrates. Electrical fields are applied to suitable portions of this panel so that information such as patterns or characters are formed on the panel. In order that the size of the gap between the substrates is uniform over the entire area of the panel, it is necessary to charge fine powder particles serving as spacers between the pair of transparent substrates. In general, two types of methods are known for charging such spacers: namely, a method called "wet method" and a method called "dry method". According to the dry method, dispersed fine powder particles are sprayed onto one of the substrates by means of a nozzle, as shown in, for example, Japanese Patent Laid-Open No. 64-76031.
The fine powder particles used as the spacers between the pair of transparent substrates have extremely small particle sizes generally ranging between several .mu.m and 10 .mu.m. The size of the spacer powder particles directly affects the performance of the liquid crystal display panel. Therefore, various restrictions are posed such as the material of the powder, distribution of the particle size, and so forth. It is also necessary that the fine powder particles are completely separated into discrete state, for otherwise the size of the gap between the transparent substrates is rendered non-uniform due to presence of secondary particles which are formed as a result of aggregation of the powder particles. Thus, industrial production of liquid crystal display devices still involves problems to be solved. Furthermore, it is required that the discrete powder particles are uniformly sprayed over the entire area of the transparent substrate without any local concentration.
Generally, the requirement for the distribution of the fine powder particles is such that, for example, 30 to 200 particles are scattered in a unit area of 1 mm.sup.2. The requirement is so severe that, when the design quantity of the fine powder particle per unit area is, for example, 50 particles, the allowable variation or fluctuation in the quantity of particles is as small as less than several to ten and several particles per unit area. Products which fail to meet this requirement are rejected as being defective. Consequently, the yield of the product is significantly lowered.
Various methods have been proposed for attaining the required uniform powder particle distribution. For instance, it has been proposed to spray the powder particles from a nozzle which is sufficiently spaced from the substrate, as well as a method in which the spray nozzle is made to revolve along a circular path, in order to attain a uniform distribution.
The condition for supplying the powder to the spraying apparatus, as well as the spraying method, is an important factor which affects the distribution of the fine powder particles. For instance, when the fine powder particles are continuously supplied from the nozzle, uniform distribution of the powder particles cannot be obtained unless fluctuation in the supply rate per unit time is minimized, even though the condition of spraying is strictly controlled.
To achieve uniform spray of powder particles, therefore, a specific powder particle supplying apparatus is used such as a screw feeder, a table feeder or a fluidized bed feeder.
Such a known powder supplying apparatus, however, essentially has a comparatively large movable part which optimumly operates when powder particles of large particle sizes are supplied in large quantity. This type of apparatus, however, is not suitable for use in cases where an extremely small rate of supply, e.g., several tens of grams per hour because in such cases it is extremely difficult to eliminate variation in the supply rate in relation to time. Consequently, the density of the powder particles is undesirably fluctuated, failing to meet the requirement for uniform distribution of the fine powder particles over the entire area.
The screw feeder type apparatus, which is one of the known powder supplying apparatus as described above, is disadvantageous in that the feeding condition undesirably varies depending on whether the screw is fully stuffed with the powder particles over its entire length or only partly stuffed with such particles as in the transient period immediately-after start up of the supply or immediately before termination of the supply. Consequently, this type of powder supplying apparatus tends to lower the yield particularly when the production specifications pose strict requirements. This problem would be overcome if the products which are produced in such transient periods are disposed of, but such a measure leads to wasting of a large amount of powder which is quite inconvenient particularly when the powder is expensive.